1. Field of the Invention
The present invention relates generally to spool valves and, in particular, to lock valves for marine steering systems.
2. Description of the Related Art
Lock valves are conventional components of marine steering systems. Such lock valves include a valve spool which is reciprocatingly received in a valve spool bore in a body of the valve. The lock valve has ports connecting it to a helm which steers a marine vessel as well as ports connecting it to a steering actuator, typically a hydraulic cylinder.
A problem may occur when the fluid flow pumped from the helm differs from the fluid flow returning to the helm. This may occur in certain conditions including in situations where the steering cylinder is unbalanced. One solution to this problem has been to provide a partial return to tank to allow pressure relief in such a situation.
One earlier related patent is U.S. Pat. No. 4,669,494 issued on Jun. 2, 1987 to McBeth which discloses a hydraulic lock valve for marine steering with partial return to tank.
Another related patent is U.S. Pat. No. 6,579,072 issued on Jun. 17, 2003 to Trousil et al. which removes the need for a separate return port for the tank passageway and makes the valve easier and less expensive to manufacture.